People often use portable computers in case of conference, speech, teaching and the like. In these situations, a document, such as a PPT document, is often opened and one of pages therein is displayed to be discussed and explained for several minutes or even more than ten minutes. Based on such application situation, the inventor of the present invention finds much power consumed with the display method of the prior art, because the computer operates only for displaying one page of the PPT during a long period of time, which is very wasteful in view of energy saving.